Coronary artery disease is a disease of the coronary arteries, which causes decreased blood flow to the heart muscle. Individuals with coronary artery disease (or other conditions) may experience myocardial ischemia, which is an imbalance between myocardial oxygen demand and myocardial oxygen supply. In some cases, myocardial ischemia may result in irreversible cardiac cell death (e.g., cell necrosis), a result commonly referred to as a myocardial infarction. Acute myocardial infarction (AMI) is the acute phase of a myocardial infarction (MI), during which cell necrosis occurs. Along with coronary artery disease, other conditions also may result in myocardial ischemia and/or AMI. For example, myocardial ischemia may result from cardiovascular disease, ischemic heart disease, pulmonary heart disease, hereditary heart disease, hypertensive heart disease, inflammatory heart disease, valvular heart disease, atherosclerosis, tachycardia, hypertension, hypotension, thromboembolism, compression of a blood vessel or artery (e.g., by a tumor), foreign matter within the cardiovascular system, sickle cell disease, and/or other causes. Minimizing the time to diagnose an ST segment elevated AMI (STEMI) and to provide treatment is critical to preventing damage to the cardiac tissue and death. Diagnoses and treatment within the first hour after a STEMI AMI event have been shown to abort MI (See JAMA, GERSH, Bernard J., “Pharmacological Facilitation of Primary Percutaneous Coronary Intervention for Acute Myocardial Infarction,” JAMA, Feb. 23, 2005, Vol. 293, No. 8, pages 979-986, and GOLDBERG, Robert J., “Duration of, and Temporal Trends (1994-1997) in, Prehospital Delay in Patients With Acute Myocardial Infarction: The Second National Registry of Myocardial Infarction,” Archives of Internal Medicine, October 1999; Vol. 159, pages 2141-2147.).
Acute coronary syndrome, commonly referred to as a “heart attack,” refers to the series of events associated with coronary vessel closure or occlusion. The series of events may proceed from stable angina (e.g., chest/arm pain or dyspnea with exertion) to unstable angina (e.g., chest/arm pain at rest or increasing frequency of angina). Acute coronary syndrome may then proceed to an AMI and death, in some cases. Myocardial ischemia is present at the onset of acute coronary syndrome, and may indicate the potential for an impending AMI.
Myocardial ischemia may be transient, sub-lethal or persistent. Transient myocardial ischemia may have a relatively short duration (e.g., minutes), with prompt reperfusion of the coronary blood vessels. Accordingly, cell necrosis does not typically occur with transient myocardial ischemia. Sub-lethal myocardial ischemia may have a significantly longer duration (e.g., weeks or months), although it also is characterized by subsequent reperfusion and no cell necrosis. Persistent myocardial ischemia, during which no reperfusion occurs of the coronary blood vessels, may result in cell necrosis and death.
An individual may experience physical symptoms warning them of the onset or presence of transient, sub-lethal or persistent myocardial ischemia. These warning symptoms may include, for example, chest and/or arm pain (e.g., stable angina or unstable angina), shortness of breath, nausea, vomiting, palpitations, sweating, weakness, fatigue, anxiety, and/or one or more other physical symptoms. Through education, many people realize that they should promptly seek medical attention at the onset of such physical symptoms. However, a significant number of individuals experience non-specific symptoms or “silent” myocardial ischemia, during which they do not perceive any physical symptoms of myocardial ischemia. Accordingly, an acute coronary syndrome for such an individual may proceed to an AMI before this individual becomes inclined to seek medical attention.
When medical personnel (e.g., emergency room personnel) are presented with a patient who exhibits symptoms of myocardial ischemia, the conventional standard of care includes applying current AMI detection apparatus and methods. Typically, this includes monitoring the bioelectrical impulses from the patient's heart using an electrocardiogram (ECG), and looking for certain characteristics of the ECG waveform. A typical ECG may have as many as twelve leads, and this system may have only about 50% sensitivity to the detection of an on-going AMI at the time the patient first arrives at the emergency room. Accordingly, the patient's heart rate and certain cardiac biomarkers, present in the blood, also may be monitored. However, cardiac biomarkers associated with severe ischemia typically are not produced in significant quantities until cell necrosis has occurred, and it may take several hours for the biomarker concentrations to reach measurable levels.
When an impending or current AMI is diagnosed, therapies may include thrombolytic therapy and/or administering aspirin, beta-blockers, nitrates, and/or statins, among other things. These therapies, when administered promptly and properly, may minimize or stop cell necrosis. Accordingly, an individual experiencing myocardial ischemia, particularly persistent myocardial ischemia, is well advised promptly to seek medical attention, in order to reduce the occurrence or severity of cell necrosis and, in some cases, to avoid death. However, a patient may delay early action due to denial, psychological factors, and/or physical factors. In addition medical personnel may delay early treatment because of the time required for running diagnostic tests.
For at least the foregoing reasons, a need exists for methods and apparatus to detect the onset or presence of myocardial ischemia, and to provide more accurate and timely cardiac monitoring in the presence of myocardial ischemia.